1. Technical Field
The present invention relates generally to cell-based networks and more specifically to a method and apparatus for policing incoming cell-based traffic at network nodes.
2. Description of the Related Art
Traffic management in communications networks requires controlled use of network resources to prevent network traffic bottlenecks. In particular, when network resources are allocated to more connections or traffic than the resources can effectively support, network performance for users degrades (i.e., buffers start to overflow and delays increase beyond acceptable levels). Therefore, it is necessary to manage resource utilization and control network traffic so that the network can operate with acceptable efficiency even at during peak traffic periods.
Traffic management is a complex task in cell-switched ATM networks, due to the random nature of traffic arrival patterns and resulting unpredictable contention for network resources. In simple terms, cell-switched networks include a queue associated with each communication link at every switching node in the network. As the arrival rate at a link approaches its transmission rate capacity, the buffer queue length grows dramatically. As a node becomes congested, buffers start to overflow and cells that arrive at a time the buffer is full are discarded.
In accordance with higher level protocols, all of the cells within a message (stream) having dropped cells are eventually required to be retransmitted by an upstream node (or by the source), further increasing the traffic load on the network. As the number of retransmissions increases, more nodes become congested and more cells are dropped. Eventually, the network can reach a catastrophic state in which most of the packets in the network are retransmissions. Thus, the challenge in cell switching is to develop a framework that maximizes the utilization of network resources while controlling the traffic flow in the network so that the temporary periods of overload that occur due to the stochastic nature of the traffic do not turn into sustained periods of congestion.
One type of traffic control used in ATM cell-switched networks is flow control that regulates the rate at which the sender transmits cells (i.e., traffic shaping), to match the rate at which the destination station can receive data. To prevent the destination station from becoming overwhelmed, call admission control (CAC) is performed when a new connection request is received at the network. CAC is a call admission procedure that is executed to decide whether or not to accept or reject the call. A call is accepted if the network has enough resources to provide the Quality of Service (QoS) of the connection request without affecting the existing connections. However, call admission is not sufficient to prevent congestion because users may not stay within the connection parameters negotiated at the call setup phase. Several possible reasons for failing to remain within negotiated connection parameters include users not knowing or underestimating the connection requirements; user equipment malfunction; users deliberately underestimating their bandwidth requirements in order, for example, to pay less; or users deliberately trying to sabotage the network.
Thus, the network must include functionality to ensure that sources stay within their connection parameters negotiated at the connection setup phase. This function, referred to as traffic policing or usage parameter control (UPC), resides at the access point to the network. A policing function must detect a nonconforming source as quickly as possible and take appropriate actions to minimize the potentially harmful effect of the excess traffic. This is achieved transparently to conforming users, in that the traffic generated by such sources are not affected.
Actions that a policing function can take when a source is detected to be nonconforming include: Dropping violating cells; marking violating cells to be processed differently than conforming cells during periods of congestion; and adaptively controlling the traffic by immediately informing a source when it starts to violate its contract.
Enforcing traffic policing is thus a key to prevent congestion in ATM networks. To this end the leaky bucket scheme was first introduced by S. Akhtar, in a MS Thesis, xe2x80x9cCongestion Control in a Fast Packet Switching Networkxe2x80x9d, presented at Washington University, in 1987. The general idea behind this approach is that a cell, before entering the network, must obtain a token from a token pool. An arriving cell consumes one token and immediately departs from the leaky bucket if there is at least one available in the token pool. Tokens are generated at a constant rate and placed in a token pool. There is an upper bound on the number of tokens that can be waiting in the pool, resulting in discarding or marking tokens arriving after the token pool is full. The size of the token pool imposes an upper bound on the burst length and determines the number of cells that can be transmitted back-to-back, controlling the burst length. The maximum number of cells that can exit the leaky bucket is greater than the pool size, since while cells arrive and consume tokens, new tokens are generated and placed in the pool. A version of the leaky bucket scheme has been standardized by the ATM Forum Technical Committee in their Traffic Management Specification whose version 4.0 was published in April 1996 under reference af-tm-0056.000. The policing algorithm is referred to as Generic Cell Rate Algorithm (GCRA).
Though simple, GCPA must be run for every arriving cell so that the cell can be declared as conforming or nonconforming against the negotiated connection parameters. Moreover, GCRA must be run twice since two types of traffic behavior are considered. Traffic policing is enforced for Peak Cell Rate (PCR) so that no burst of cells can arrive in too short a period of time while a Sustainable Cell Rate (SCR) must be enforced over an extended period of time.
A cell is identified as conforming when it passes both tests. Considering the high transmission speeds encountered on ATM networks, with transmission lines up to 2.4 Gigabits per second (2.4xc3x97109 b/s) (i.e., one cell arriving every 176 Nanosecond (176xc3x9710xe2x88x929 s)) at switching nodes, GCRA is implemented in hardware. Thus, every simplification in the implementation of GCRA is valuable in facilitating its hardware implementation and reducing the number of steps and operations to be able to cope with the very high performances required at entry points of ATM networks.
A method and system for enforcing a transmission control parameter for a virtual connection comprising multiple cells within an input port are disclosed herein. The virtual connection has a nominal inter-cell arrival rate increment. Each of the cells are associated with a theoretical arrival time and an early arrival limit, wherein a difference of the early arrival limit from said 10 theoretical arrival time delineates a lower boundary of a good zone, while the theoretical arrival time minus the sum of the early arrival limit and the nominal inter-cell arrival rate increment delineates an upper boundary of the good zone. The method includes determining an actual arrival time for an incoming cell from the cells that make up the virtual connection. Next, the actual arrival time is compared to the upper boundary and the lower boundary to determine whether or not the actual arrival time falls within the good zone. In response to determining that the actual arrival time falls within the good zone, the incoming cell is identified as conforming to the transmission control parameter. The method and system of the invention allow to use a finite counter for measuring elapsed time at entry ports of ATM switching nodes, a mandatory feature in order to enforce policing, while avoiding having to manage the overflows of the counter.